[vorbis-dev] slightly different audio output

[Pattara Kiatisevi]

On Tue, 9 Apr 2002, Segher Boessenkool wrote:

> > -I debuged the code and found that mdct_backward() returns quite different
> > values between float and int setting. (before MDCT, there error is almost
> > off-by-one, at most 1%).
>
> Yes.  The integer mdct code in libvorbis is pretty bad.  There are
> comments in the code that say so, even :)

Oh, I thought it would work fine with this 32x32=64 feature.

>
> > -I'm using MDCT code from RC3 (not sure if the code in CVS is better?). I
> > have tried with 14-bit TRIGBITS and then modified the code to include
> > Segher's magic union:
> > http://www.xiph.org/archives/vorbis-dev/200202/0081.html. And run it at
> > TRIGBITS = 14, 20, 25, 30.
>
> Do you use rounding or truncating? ...

Hmm...

tatic inline int himul(int x, int y) {
  union magic magic;
  magic.whole = ((long long)x * (long long) y) >> TRIGBITS;
  return magic.halves.lo;
}

Do you have suggestion?

<p>>
> > -the output from mdct_backward() seems to converge to a set of values when
> > increasing TRIGBITS but it doesn't converge to the result from floating
> > point version though.
>
> ... If you use truncating, the libvorbis mdct algorithm is pretty bad.
>
> > -has this int implementation of MDCT been proved to yield the same (or
> > almost the same) result with floating point version when using at high
> > TRIGBITS? Maybe my workaround is not correct..
>
> No, but it _has_ been proven to yield bad results (in some cases).
>
>
> If you have 1024-or-so-bit integer math, converting floating point code
> to integer code is easy.  In all other cases, you need some
> serious engineering work, to make it high-quality.
>
> I'm sure you'll enjoy it,

ok, for this Ogg-on-a-Chip project I think we could stay with this quality
for a while. Hmm, tomorrow I will do the blind test if people can
differentiate between the output from floating point and this
quite-poor-quality integer decoder. :)

Anyway, thank you Dan Conti, your FullPlay code did help us and best luck
to Xiph's crew about the embeddable decoder. Vorbis rulezzzzzzz....

Thanks,
Pattara

<p>>
> Segher
>
>
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